1. Field of the Invention
This invention relates to thermal dye transfer printing, and more particularly to dyes used in a thermal dye transfer printing construction. The dyes comprise a specific type of mixture known as a eutectic mixture. The eutectic mixture has at least two components.
2. Background of the Art
Various dyes are used in thermal transfer systems. Dyes are used in both thermal mass transfer systems, and thermal dye transfer systems. Dye groups described in the prior art are generally characterized as relatively sublimable disperse dyes or solvent dyes. Dyes are generally used singly or as a combination for a monochrome color. Some patents list dyes used in combination, but relatively little information is given to the composition and properties of the dye mixtures.
Dyes are usually described as being dissolved in a solvent with the binder resin and coated onto a substrate to form an ink layer on the substrate. The dye is often described as subliming under the action of the heat energy of the thermal head, and transferring to an image receptive sheet.
Dyes are also described as being suspended within the binder in the form of particles. To facilitate sublimation, the dyes usually have a low molecular weight of about 100 to 750. Criteria for selection of dyes include sublimation temperature, hue, weatherability, solubility of the dye in ink compositions or binder resins, and other factors. The dye is usually present in an amount which is dependent upon the degree of its transfer at the sublimation temperature, and the covering power in the transferred state.
The broad technical area of imaging art contains a number of disclosures of dye- or dye former-containing eutectics, many of them used in certain imaging procedures. Frequently, mention is made in the literature of eutectic compounds or eutectic complexes. There are no such materials as eutectic compounds or complexes in the true technical sense normally understood by a chemist. Eutectics are definitely mixtures, not compounds, of two or more chemically distinct entities. Furthermore, a solid eutectic contains separate crystals of each of the mixed entities, not a mixture at the molecular level.
U.S. Pat. No. 4,614,682, entitled "Thermosensitive Image Transfer Recording Medium" discusses a thermosensitive recording medium comprising a support material and a thermofusible ink layer formed thereon, which thermofusible ink layer comprises a dye component, a binder agent, and a pigment having needle-like crystal form, which is dispersed in a network form throughout the thermofusible ink layer. Dye components are specifically described as, "it is preferable that the eutectic temperatures of the dyes to be used with a binder agent be in the range of 50.degree. C. to 140.degree. C., although the eutectic temperatures vary depending upon the binder agent to be used in combinations." Claim 7 details "A thermosensitive image transfer recording medium as claimed in claim 1, wherein the eutectic temperature of said dye component in combination of said binder agent is in the range of 50.degree. C. to 140.degree. C." It appears this refers to a eutectic combination of a dye and a binder.
The same mentioned patent refers to the dye as being of a smaller particle size than the needle-like pigments to be used, and that the dye be in a dissolved state.
Japanese patent publication JP 60-056590 assigned to Mitsubishi Electric Corp. describes a reusable heatsensitive recording sheet which includes a layer containing: (a1) dye; (a2) material lowering the melting point of (a1); (a3) material dissolving (a1) and (a2) at elevated temperature; (a4) a surfactant with melting point of 40.degree. to 100.degree. C. Preferably the mixture of (a1) and (a2) is what is described as eutectic or cocrystalline material. The mixed ratio of (a1) and (a2) is 1:10 and 10:1 Dye (a1 ) is preferably an anthraquinone or azo disperse dye. Compound (a2) is, e.g. p-nitrobenzaldehyde, stearamide, methyl-4-tert-butylphenol, etc. Material (a3) is, e.g. glycerin, diethylene glycol, triethylene glycol, etc. Surfactant (a4) is, e.g., an ester of a long chain fatty acid. The mixture (a1)-(a4) is contained in a polymeric binder. The advantages of this invention are good sensitivity, good gradation properties, and reuse.
Matsushita Electric Corp. Japanese patent abstract JP 59-93389 speaks of a color sheet material for thermal transfer with particles containing at least two kinds of coloring material. The particles contain at least one of a basic subliming dye and a disperse subliming dye. Mention is made of microencapsulation of the dye, but it is not clear whether this refers to the combination or to the individual dyes. No reference to eutectics appears in the abstract.
Many Ricoh patent publications (e.g. JP 62-135388, JP 62-130877, JP 58-211493, JP 57-201693, JP 57-014094, JP 58-211493) speak of eutectics or eutectic compounds in connection with thermal leuco dye imaging systems. Work at Fuji (A. Igarashi and T. Ikeda, Proc. 1st International Congress on Advances in Non-Impact Printing Technologies, 1982, p. 886) definitely shows occurrence of true eutectics in some constructions. However, in contrast to the purely physical process of thermal transfer in our invention, the melting of the eutectic in these systems is used to trigger a chemical reaction which results in color formation from the colorless leuco dye.
A similar situation arises in connection with thermal diazo imaging systems. Work at NTT (H. Sato, K. Sukegawa and Y. Ooba, J. Imaging. Technol., 10, 74 (1984); H. Sato, Y. Ooba and S. Sugawara, ibid., 11, 137 (1985)) has established the importance of both binary and ternary eutectics in these systems, but again this is a chemistry-triggering situation.
Several patents concern mixtures of dyes selected for hue adjustment. The abstracts make no mention of eutectics. Mitsubishi Chemical Industries (JP 61-148096) claims a sublimation transfer recording material giving pure black images from a mixture of matched yellow, cyan and magenta dyes. U.S. Pat. No. 4,401,692 assigned to Hoechst concerns a transfer print carrier for printing on polymers having a mixture of blue and red to yellow, readily sublimable, disperse dyes giving fast black dyeings. Bayer (DE 3537257) claims a mixture of specific azo and anthraquinone dyes for selectivity dyeing polyester in polyester-cotton blends by the thermosol or HT steam process. The azo is present at 90 to 99.5 wt %.
Eutectic mixtures of dyes have been investigated in connection with the non-additivity of dye adsorption isotherms for the dyeing of fibers with dye mixtures (A. Johnson, R. H. Peters and A. S. Ramadan, J. Soc. Dyers Colour., 80, 129 (1964), but this appears to be quite unconnected with the present invention. Similarly, patents on eutectic dye carriers (e.g. U.S. Pat. No. 3,925,013 and U.S. Pat. No. 3,787,181) where the dye is not part of the eutectic also appear to be of no real relevance.
Several Ricoh patents (e.g. JP 58-065441, JP 57-122040, JP 56-142536) claim electrophotographic elements with "eutectic crystal complexes" of a pyrilium dye, polymer and charge transport material, apparently analogous to Kodak work showing formation of a complex of thiapyrylium dye and polycarbonate (W. J. Dulmage et al. J. Appl. Phys., 49, 5543, (1978). Similarly Japanese patent publication JP 60-044553 (examined JP 87-04182) discusses a photoconductor sensitizing dye disclosed as a eutectic of a merocyanine dye and an organic electron acceptor.
Eutectic mixtures of compounds are cited in the patent literature that discusses eutectic compounds related to liquid crystal compounds, pharmaceuticals, perfumeries, and dye carriers for textile printing.
Japanese patent publications listing the use of anthraquinone dyes in a thermal transfer composition are JP 61-227093, 61-035993, 60-151097, 60-253595, 60-131292, 60-131293, 60-131294, 60-172591, 60-031559, 60-053563, 59-227948, 60-217266, 59-091644, 59-000221.
Japanese patents listing the use of azo dyes in a thermal transfer construction are JP 51-112993, 61-227091, 61-227092, 61-224595, 61-119786, 61-144388, 58-111176.